The tertiary structures of antibody molecules are being investigated with the main purpose of elucidating the nature of the specific combining site of the antibody molecule and its relationship to the whole antibody molecule, to the subunit structure of the antibody, to the amino acid residues in the site, and to the amino acid sequences involved in determining specificity. Correlations are being sought between the primary amino acid sequences and the tertiary structures of antibody molecules for different antibodies directed against the same haptenic group and for antibodies directed against various haptenic groups. Efforts are concentrated on that part of the sequence involved in the binding sites. To achieve this goal homogeneous antibody preparations are being studied rather than the heterogeneous preparations which have been studied in the past. Studies are being done by subjecting homogeneous antibodies to controlled chemical alterations so that reactions taking place in the binding region can be distinguished from reactions taking place elsewhere on the molecule. The amino acid residues composing the binding region are being determined, as well as the position of these residues composing the binding region are being determined, as well as the position of these residues in the polypeptide chains which make up the protein.